This invention relates to the heat treatment of metals, and more particularly to a novel process for quenching metals.
In order to modify the physical properties of metals and their alloys, various methods of heat treatment have been devised in which the metal is heated to a given elevated temperature and then cooled. Generally the cooling takes place at a relatively rapid rate, and such cooling is commonly referred to as "quenching". Quenching is accomplished by immersing the hot metal in a liquid bath, usually water or oil.
Water alone produces very rapid cooling and is not suitable for quenching many types of steel, for it produces excessive strains which warp and crack the steel. Hydrocarbon oils provide a relatively slow rate of cooling which is desired to produce certain physical properties, such as ductility, in steel. However, the slower cooling rate provided by oil quenching, although it prevents excessive strains in the metal, often prevents development of the desired hardness.
It, therefore, is desirable to provide quench liquids which will cool the metal at rates similar to oil, or intermediate between oil and water, while achieving the greatest degree of hardness without warping or cracking the steel. To this end various aqueous solutions and dispersions of organic compounds have been proposed as quenching fluids. Such aqueous solutions and dispersions approach the quenching properties of oil, but without the disadvantages of the fire, smoke and fumes associated with the use of oil.
For example, U.S. Pat. No. 3,220,893 discloses a liquid quenching medium comprising an aqueous solution of a liquid oxyalkylene polymer having both oxyethylene and higher oxyalkylene groups and a molecular weight of 12,000 to 14,000. Such compounds are also referred to as polyether polyols and poly(oxyethylene-oxyalkylene)glycols. According to the patent, the oxyalkylene polymers have the characteristic of decreasing in solubility as the temperature of the quenching bath is increased, as when red hot metal is introduced therein. The oxyalkylene polymers are said to form a covering over the metal surface to the exclusion of the water component of the bath. The polymer layer is said to be an excellent heat conducting medium which operates at a high rate, and thus use thereof requires relatively short quench cycle time, which results in minimum interval stress and distortion of the metal, while at the same time imparting uniform hardenability of the metal.
Such nonionic oxyalkylene polymer quenchants control cooling rate of the metal not only by such inverse solubility with temperature, but also by increasing the viscosity of the water in which they are dissolved. Accordingly, it has been the practice to use large amounts of polymer, e.g. 10 to 15% by weight. In use, such relatively concentrated baths suffer from the disadvantage of "drag out", i.e. the polymer is removed with the metal being quenched resulting in undesirable changes in bath viscosity and the necessity for treatment of resulting effluent from washing of the quenched metal to remove adhering polymer. Very high molecular weight nonionic polyoxyalkylene glycols can be used, but the increase in viscosity they cause makes them impractical.
It has been suggested to increase the quench cycle time by use of organic compounds which cause the formation of a vapor blanket about the metal during the quenching operation. An example of such an organic compound is a water-soluble polyacrylate, such as sodium polyacrylate, the use of which compound in a quenching bath comprises the subject matter of U.S. Pat. No. 4,087,290.
Quenching of metal using as the quenching medium an aqueous solution of a water-soluble polymer of a C.sub.2 -C.sub.4 olefin having a heterocyclic side group composed of carbon, nitrogen and oxygen is suggested in U.S. Pat. No. 3,902,929.
In copending application Ser. No. 561,980, the specification of which, as well as that of earlier filed application Ser. No. 495,679 is incorporated herein by reference, there is disclosed a quenching process employing a quenching bath comprising an aqueous solution or dispersion of a substituted oxazoline polymer, such as polyethyloxazoline, which is capable of providing quenching effects similar to those produced by oil quenchants, as well as quenching times between those provided by oil and water.
An object of this invention is to provide a novel metal quenching process employing a quenching bath which can be varied in composition to provide a broad range of quenching rates from that of oil toward that of water.
Another object of this invention is the provision of novel quenching baths which provide for slower quenching rates at the temperature where martensite transformation starts resulting in improved physical properties in the quenched metal.
A further object of the present invention is to provide novel quenching baths which provide cooling rates comparable with those based on aqueous solutions or dispersions of substituted polyoxazolines, but using reduced amounts of such compounds and consequently providing reduced costs.
Still another object of this invention is the provision of concentrates which can be diluted with water to form quenching baths of reduced quenching rates.